자바 에서 JDK SPI 소스 코드 분석

com.union.jd.ASpiServiceImpl
com.union.jd.BSpiServiceImpl
com.union.jd.CSpiServiceImpl

ServiceLoader<SpiService> serviceLoader = ServiceLoader.load(SpiService.class);   
//ServiceLoader实现了Iterable，所以可以直接使用增强for遍历
for (DriverService driverService: serviceLoader){
	System.out.println(driverService.getName());
}

public final class ServiceLoader<S>implements Iterable<S> {...}

private static final String PREFIX = "META-INF/services/";

//入参为接口的全限定名
public static <S> ServiceLoader<S> load(Class<S> service) {
    //获取当前线程上下文类加载器
    ClassLoader cl = Thread.currentThread().getContextClassLoader();
    //调用重载方法
    return ServiceLoader.load(service, cl);
}
//重载方法：参数1为接口的全限定名、参数二维当前线程上下文类加载器
public static <S> ServiceLoader<S> load(Class<S> service,ClassLoader loader)
{	//创建ServiceLoader对象
	return new ServiceLoader<>(service, loader);
}
//ServiceLoader构造方法
private ServiceLoader(Class<S> svc, ClassLoader cl) {
    //校验Class对象是否存在，赋值给变量service
    service = Objects.requireNonNull(svc, "Service interface cannot be null");
    //当"线程上下文类加载器"为null则获取"系统类加载器"，赋值给变量loader
    loader = (cl == null) ? ClassLoader.getSystemClassLoader() : cl;
    //security沙箱相关
    acc = (System.getSecurityManager() != null) ? AccessController.getContext():null;
    //调用reload方法
    reload();
}
//reload方法
public void reload() {
    //清空缓存的实例对象
    providers.clear();
    //创建LazyIterator内部类对象，赋值给lookupIterator属性，iterator()遍历就是对该属性遍历
    lookupIterator = new LazyIterator(service, loader);
}

private class LazyIterator implements Iterator<S> {...}

//ServiceLoader 的迭代器方法,lookupIterator是LazyIterator的实例
public Iterator<S> iterator() {
    return new Iterator<S>() {
		...
        public boolean hasNext() {
            ...
            return lookupIterator.hasNext();
        }

        public S next() {
            ...
            return lookupIterator.next();
        }
		...
    };
}

public boolean hasNext() {
    ...
    return hasNextService();
    ...
}

public S next() {
	...
	return nextService();
	...
}

private boolean hasNextService() {
    ...
}

private S nextService() {
    ...
    //通过上面传的类加载器，反射获取当前实现类的class对象
    c = Class.forName(cn, false, loader);
    ...
    //类型转换
    S p = service.cast(c.newInstance());
    //添加到缓存中
    providers.put(cn, p);
    //返回实例对象
    return p;
    ...
}